This invention relates to implantable devices including but not limited to tissue stimulators having measurement capability for determining impedance measurements and is particularly well suited to measure long term edema variations within a living body.
Impedance monitoring has been used for determination of numerous physiologic conditions within the body with implanted devices and has been used in external monitoring devices as well. It is commonly understood that Transthoracic Impedance measurements give a good indication of the level of edema in patients. Even as far back as 1971, in an article entitled "Transthoracic Electrical Impedance as a guide to Intravascular Overload" by Berman et. al. (Archives surgery, V102 P61-64 Jan. 1971), electrical impedance methods have been used to document the accumulation of fluid in the living tissue.
What's important about long term impedance measurement and noting changes therein is that it is an valuable clinical indicator of the health of the living body which has heretofore been unavailable to physicians in a very useful form.
While edema is a sign of many other conditions it is also a sign of the failing heart circulation which is our first concern. There are several mechanisms or diseases that can cause or affect edema. In general edema is a failure or over response of homeostatic process within the body. The body normally prevents the build up of fluids by maintaining adequate pressures and concentrations of salt and proteins, and by actively removing excess fluid. If a disease affects any of these mechanisms the result can be edema. This includes heart failure, left sided MI(Myocardial Infarction), high blood pressure, altitude sickness, emphysema (all which affect pressures), and cancers that affect the lymphatic system, diseases which disrupt the protein concentrations, . . . etc. The list is large. The point is that by providing an adequate monitor of edema we can provide a physician and his patient with a better tool to manage disease.
Unfortunately, ordinarily the first indication that a treating physician would have of the occurrence of edema is very late in the disease process when it becomes a physical manifestation with swelling or breathing difficulties so overwhelming as to be noticed by the patient who then proceeds to be examined by a physician. For a cardiac heart failure (CHF) patient, hospitalization at such time would likely be required. A device and system as proposed in this application can obviate the need for proactive hospitalization simply to monitor a patient's progression of edema as hospital stays are discouraged whenever possible under the emerging health care delivery system in the world today. Therefore a strong incentive or need exists for devices that can allow a patient to be monitored for disease symptoms over a long term without requiring hospitalization and allows for out-of-hospital intervention when symptoms, in this case, due to edema suggest it.
Additional need for this type of invention is found in the article "EFFECTS OF PREHOSPITAL MEDICATIONS ON MORTALITY AND LENGTH OF STAY IN CONGESTIVE HEART FAILURE," by Wuerz and Meador, ANNALS OF EMERGENCY MEDICINE, 21:6, June, 1992, pp 669-74, in which it is demonstrated that early pre-hospital treatment can save lives. A device that establishes new indications before the patient can be hospitalized by allowing for a readout of accumulated and trend data can be seen as an improvement in the tools available to save lives from CHF.
There are numerous devices and teachings which describe or are capable of making impedance measurements including U.S. Pat. Nos. 5,534,018, 5,271,395, 5,370,6665, 5,233,985, and 5,282,840. Likewise, there are numerous mentions of the use of impedance for determination of levels of edema in the literature without the use of long term implantable devices which usually require significant and expensive monitoring, attention, and effort. Such monitoring and effort may not be available before hospitalization. However, there currently are no impedance measurement devices suitable for providing an indication of when an edema is about to become a serious problem for a patient, nor for closely monitoring it over a protracted period of time. Consequently, as patients become out of titer on their edema management medication (diuretics, for example) or after they unknowingly or unintentionally eat salty foods, hospitalization may be enhanced or possibly avoided through the use of a device as described herein. Thus healthcare resources can be used more efficiently and effectively to help CHF patients in particular as well as other patients which can benefit from similar information availability.
It should be noted that adaptations of some technologies for performing some patient monitoring functions are developed as evidenced by U.S. Patents to Yomotov, et al, Nos. 5,313,953 and 5,411,031, incorporated herein by reference in their entirieties. However as yet there has not been acceptance of implantable devices for monitoring purposes alone, and certainly none for simply monitoring edema per se, nor for edema as a partial indicator of health, nor for use of edema monitoring as an adjunct indicator for therapy modification. Accordingly it is believed there is a large need for the teaching of this invention.
Additional measurements--Respiration rate.
Additional data can be found in the respiration rate which can be monitored as a separate and independent indicator of edema onset and particularly of pulmonary edema or increased lung water, since patients are known to breath quickly when their lungs fill with fluid. It is known, for instance that in extreme cases of pulmonary edema breathing rates of 60 per minute can occur. Because it would be impossible to clinically measure slight changes in long term breathing rates, for example a 5.5 to 5.8 breath per minute change, but such monitoring is possible with an implanted device, by including a processor for sampling the rate of breathing and comparing it to next sampled rates over time, substantial clinical benefit can be obtained by making this data available.